


Laying Claim

by Mercy_Run



Category: Undertale (Video Game)
Genre: And cheesy pickup lines, First Time, Fontcest, Interesting use of Gaster's multiple hands, Language, M/M, Semi-Public Sex, Size Difference, Smut, Workplace Sex, messy sex, puns
Language: English
Status: Completed
Published: 2017-09-17
Updated: 2017-09-17
Packaged: 2018-12-31 00:41:12
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 2,676
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/12120810
Author URL: https://archiveofourown.org/users/Mercy_Run/pseuds/Mercy_Run
Summary: Gaster could only take so much teasing before even his patience and steely control failed him. Especially when it concerned his infuriating lab assistant. He would make sure that Sans’ had an attitude adjustment.  In the most pleasurable of ways.NSFW Blog: mercy-run.tumblr.com





	Laying Claim

**Author's Note:**

> Secret Santa gift for Zed last year. <3

The Royal Scientist’s right socket twitched as he attempted to focus on his work and not the source of his frustrations. Gaster had much more important things to do than indulge in his own desires. Like the ever-present problem of being trapped by the barrier. You’d think that would be more than enough to clear his mind but ever since the monster had hired his new assistant; his life had been plagued with new feelings. Being in such a prestigious field had Gaster carrying himself with dignity and grace when in the public eye. His self control and tolerance for things that irritated him was abnormally high.

But the scientist couldn’t ignore the short skeleton’s flirting, no matter how awful the humor was. Sans had been quiet the first few months on the job, likely intimidated by him like a lot of others were. But gradually, the small skeleton opened up in the…most odd of ways.

“do you have eleven protons? ‘cause you’re sodium fine!”

The first time, Gaster had let out a startled laugh and the way Sans’ eye-lights trembled with awe had struck him straight to his soul. It was like the taller monster had opened the floodgates after that.

“i prefer my ribosomes bound tight.”

“are you made of copper and tellurium? because you are cu-te!”

“didn’t you know chemists do it periodically on the table?”

He had dropped the vial of extract he had been working at that one. Gaster had tried to reason that Sans was just trying to get him to react again. Maybe he liked the out of character laughter he received. That was what comedians lived for, after all. That his assistant was both an aspiring scientist and a joker made little sense to him.

Yet, over time Gaster began to notice things about Sans. That expression he had first glimpsed was one of his favorites on the other monster. But he had also noticed that even though the other monster pulled pranks and joked a lot, the skeleton had a fierce love for his little brother and went out of his way to make sure other people were taken care of. That last little tidbit had been discovered on accident. Gaster had been dozing at his desk and he woken to find Sans tidying up from where the reports had fallen when he had dozed off. The other had covered him with the heavy coat he kept in the lobby for his rare trips to Snowdin.

That act of kindness had been done when Sans hadn’t realized he had been watching

And his assistant hadn’t brought it up or tried to use it against him in anyway. Not even to joke about it. So he had began to watch Sans more closely and realized this wasn’t the first time nor the last that he would do such things.

It had…drawn him and made Gaster acutely aware of Sans.

At some point, whether from that or from the flirting, the Royal Scientist began noticing something very appealing about his assistant. Gaster was used to all but the King and Queen being shorter in stature to him. It was something he had always secretly enjoyed. Whether the source was from the sense of extra authority it gave him or otherwise; it was something that he looked for in a partner when the mood struck. While most monsters were shorter than him, Sans was especially so. Furthermore, unlike most monsters; the skeleton didn’t act meek around him. In fact, the other seemed set on riling him at everyone opportunity.

It was maddening.

With the cheesy puns and pick-up lines on top of it, Sans was driving the scientist insane in more ways than one. Gaster had given in to sexual fantasies about putting the other monster in his place in various pleasurable ways, but he was hesitant to act on it. With his small assistant, he gleaned that his desires just weren’t born from lust. If he pursued something, he wanted to make sure it was more than a scandalous work-place affair. So he had given in, despite all doubts.

Alphys had been more than eager to confirm that Sans’ interest was genuine.

The knowledge had been offered a week ago and Gaster felt like Alphys had ratted on him with how ambitious Sans was getting with his flirting. Whatever it was, the Royal Scientist could bare it no more.

On cue, there was a knock at his office door. A heavy sigh passed his teeth; he knew very well Sans wouldn’t come in if he didn’t play along. And he needed the other monster inside safely before attempting anything…

“Who’s there?” He inquired in exasperation.

“broken pencil.”

Gaster hesitated, brow furrowing. He didn’t think he had heard this one before…

“Broken pencil, who?”

“forget it, this joke is pointless.”

A startled snort had to be muffled behind his hand but Sans must have heard it regardless; opening the door and letting himself in with a big grin.

“you’re laughing.” The small skeleton pointed out, shutting the door and striding up to his desk.

“Only because it’s absurd.” Gaster murmured, though internally he couldn’t deny that it had been clever; especially for a knock-knock joke. Sans searched his expression before looking around the office, expression falling into confusion when he didn’t see any work in sight.

“uh… are you maybe taking a break? i’ve never seen your office so spotless.”

So, Gaster was still able to throw Sans off… That was refreshing because this occasion was anything but normal.

“Come here, Sans.” He said, deciding to cut right to the chase. The small skeleton blinked his sockets before walking around Gaster’s desk to stand at his side, confusion easing into nervousness. Even sitting down like this, the other monster didn’t come up to his shoulders. Pleasure warmed the Royal Scientist’s soul at that, taking in his employee’s face.

“I must address your behavior in the workplace,” the monster’s face fell in fear, “it has come to my attention that your habit to flirt with other monsters is distracting many from thei-”

“but i only flirt with you!” Sans insisted in a panic and Gaster had to use every bit of willpower he owned not to smile at the way the other skeleton had played right into his hand. A few seconds after those words had been spoken, the other’s anxiety visibly doubled along with a very alluring blue blush rising on his face.

“Do you?”

“i…i…uh…”

Hook. Line. And sinker.

Swiveling his chair to face the skeleton directly, Gaster reached forward and cupped the smaller monster’s cheek. Sans’ eye sockets were wider than he had ever seen before; a faint tremor rattling the skeleton’s bones softly. But he was leaning into the contact and not jerking away. Alphys had told him the truth, apparently. That was relieving; this would’ve ended badly if Sans didn’t hold a secret flame for him.

“What were you hoping for by flirting with me?” He questioned, voice canted low and soft.

Sans seemed incapable of stringing two words together, stuttering worse than Alphys ever had on her worst days.

“Do you want my attention so badly, dear assistant?” His thumb traced the line of the other’s teeth and gingerly applied pressure, “…pull away now if you don’t want it.”

Surprise flashed across Sans’ face before the trembling eased off. The monster remained rooted where he was, leaning into the contact enough for Gaster’s pressing thumb to part his teeth to reveal four sharp incisors.

The Royal Scientist wasted no more time, closing the distance by leaning down and conjuring his long purple tongue. The slimy appendage slid easily past Sans’ teeth and into his skull, meeting gathering blue magic and twining around it; encouraging it to take shape. A low whine escaped the smaller monster, hands coming up to grip his legs as Sans leaned into their kiss, their magic slick and growing warmer. Gaster encouraged a slower pace, enjoying how the smooth slide of their soft ecto-appendages sparked magic into his pelvis. The small eager moans had the Royal Scientist pulling back, Sans’ chasing his retreat with a hazed over expression.

A thick string of magic connected their tongues before breaking off.

“Tell me you want me, Sans. I’ll take excellent care of you.”

The tinier monster panted slightly through his fangs, eye-lights dazed, “i…oh fuck, i want you. ‘dings, please!”

Gaster needed no further encouragement, diving forward to claim the other skeleton’s mouth; a trail of mixed magic leaking down Sans’ chin. He conjured multiple glowing hands, his own grasping his assistant’s hips and lifting him into his lap. Sans gasped into his mouth as one disembodied hand pulled off his lab coat whilst another slid under his shirt, curling around his floating ribs.

There already was a small bulge glowing a faint blue in the other monster’s slacks. Gaster ground his larger pelvis into it and Sans arched against him, their tongues disengaging for the other’s cry to ring out unhindered. The conjured hand not busy fondling the small skeleton’s ribs shoved it’s fingers into the other’s mouth, keeping it open. Sans whined around the magic digits, slicking them with gooey blue magic as it stroked his tongue and trapped it.

His assistant was the very vision of debauchery…

Canting his hips into Sans’, the magic sparked from his earlier pleasure finally formed; hard and more than ready for the other monster. Sans moaned around the disembodied fingers, drool tracking down his chin as he arched into the attention, sockets widening at the sheer size rubbing against his clothed entrance.

Gaster chuckled as he distantly felt the sensations of the conjured fingers playing with the skeleton’s tongue. He directed the hand caressing Sans’ ribs to wrap around his small spine and stroke it as he reached between them to undo the monster’s pants. The cock that sprung free was small but had a decent girth, already weeping magic from the tip. A whimper was muffled with the busy fingers in Sans’ mouth as Gaster smeared the pre all over the blue head of the other’s erection.

It was too easy to lift the other monster and completely rid him of the rest of his clothing; another hand conjuring to tug off Sans’ shirt while he stripped the pants down trembling femurs. Completely nude, the skeleton monster seemed even tinier and vulnerable; causing his erection to twitch in his pants. Small hands were suddenly tugging at his belt buckle and a deep chuckle of amusement escaped the Royal Scientist.

“So eager…”

Sans’ blown eye-lights trained hazily on his expression and Gaster directed the hand in the other’s mouth to withdraw; the other’s face messy with drooled magic. The smaller monster’s hands stilled with Gaster’s pants halfway open when that same hand lowered to his puckered entrance and teased around the tight ring.

“’d-dings!” He shuddered in the scientist’s lap, assaulted by the sensations wracking his tiny body. A digit slick with Sans’ magic easily slipped inside as Gaster finished what the other monster had started; his cock springing free of it’s confines as he lowered his pants just enough to get his dick out.

Nervousness flickered across Sans’ face despite the generous attention to his spine and clenching channel at the sight. Gaster hushed the skeleton.

“I’ll go slow…”

Sans’ gaze lifted to his face and a tentative smile pulled his teeth up as he panted; reaching out and embracing his boss. Gaster could feel the happiness and pleasure radiating from the other’s soul. Yes… This was exactly how he had envisioned the other feeling. He wouldn’t let Sans go after this…

He lowered his hands down to the other’s pelvis and rocked their hips together, the slick sound of another finger working into Sans causing the scientist to murmur soft praises to the small quaking monster.

“g-gaster…! fuh…ah!” Sans was so vocal, his body seemingly unable to decide if it wanted to grind against Gaster’s fully clothed body or back into the fingers causing his soul to spark. They only withdrew when four of them were buried nearly to the knuckle, the other’s cries indicating Sans was too close.

Gaster wanted Sans to come only when his cock buried deep inside him first.

The other’s dismay was assuaged when the large head of Gaster’s cock nudged against the loosened pucker, pressing inward slowly until the walls relaxed enough for the tip to breach the tiny monster.

“ahhh!”

“Shhh…. I got you,” Gaster whispered, holding the trembling skeleton close as Sans adjusted. The squeeze against his shaft was maddening, despite the careful stretching; Sans’ walls were attempting to force him out. He wanted so much to just slam home seeking his own pleasure, but that would hurt the fragile monster.

The Royal Scientist didn’t want to harm Sans, especially like this.

“y-you…” The small monster’s breath hitched as the small amount of relaxation had Gaster inching deeper within, “c-can move now, ‘dings…”

Still, he waited; the hands he conjured from magic stroking Sans’ spine and ribs as he kissed the other breathless. Slowly, the walls relaxed and Gaster pressed all of the way inside, wringing a cry that was more desperation than pain.

“please!”

The scientist didn’t need any further encouragement. Holding the small monster close, Gaster swiveled the chair they were in and stood briefly, the movement causing Sans to cry out as it buried his cock impossibly deeper.

He pushed Sans down onto his desk, the conjured hands trapping the other’s wrists and holding it above his assistant’s head. All thoughts and words fled from both skeletons as Gaster withdrew nearly all of the way before firmly thrusting home. Another hand had to be summoned to muffle Sans’ screams of pleasure, the free one moving to stroke the smaller monster’s shaft slowly between them.

Fucking Sans was everything Gaster had imagined and even more. Even with all of the stretching, the tiny monster was still so tight. His expression, the feeling of a smaller body squirming underneath him, how much of a pleasure driving into his magic was. Sans was perfect.

And his.

Pressing the other’s small femurs up against Sans’ ribcage, Gaster groaned as his new lover screamed into the hands conjured to keep him quiet; the soft, warm walls of Sans’ passage clenching around him so tightly it nearly hurt. He dispersed the magic around Sans’ mouth. The Royal Scientist wanted to hear him…

“i-i—fuck, i’m cumming!” The cyan dick one of Gaster’s disembodied hands was jacking twitched, spurt after spurt of thick blue cum painting Sans’ bared bones, “dings! g-gaster!”

The sound of the normally cocky and cool skeleton crying his name desperately coupled with him diligently fucking into the spasming magic had Gaster tipping over the edge, ending as deeply as he could into the other’s body. The barest of growls slipped from the larger monster, cock jerking against Sans’ over-sensitive walls as rope after rope of purple cum flooded the small passage. It was the hardest and most intense orgasm Gaster had ever experienced; Sans passage quickly filling past capacity. Glowing magic gushed out of where they were joined, soaking the desk.

Gaster leaned over the smaller body of his assistant, both of them panting as they came down from their high. Basking in the afterglow.

He could feel their souls pulsing in tandem. When he opened his eyes, he discovered that Sans had fallen asleep on the cum-stained desk with his cock still buried to the root. The peaceful and happy expression there had the Royal Scientist smiling; contented.

“Don’t think you are getting away from me after this…” He murmured to the sleeping monster.

 

———————————————————————————————————

Later, Alphys email pinged her with a notification from Doctor Gaster. She opened it and adjusted her glasses before flushing a deep red.

“O-Oh m-my!”

She hadn’t expected her boss to honor their agreement if Sans ended up in a relationship with him with her help. The sight on her screen proved otherwise.

———————————————————————————————————


End file.
